I owned the Panasonic DMC-GF1 and the Olympus PEN E-P2 at the same time, but used the GF1 the most, including a three week trip to Eastern Europe. Both were usefully compact, despite the bulk of their Micro Four Thirds lenses, but I preferred the GF1's in-lens stabilization over the E-P2's one-size-fits-all body stabilization. IQ was about the same.
I dumped both for the Sony NEX-5N, which seemed to have better image quality overall and less noise with its larger sensor. I've got the Zeiss 24mm f/1.8 and Sony 18-200mm lenses for it, and am waiting on my NEX-7.
Right now I'm using the Nikon V1 more than the Sony, because I bought all the Nikon 1 lenses, plus the FT-1 adapter and have been playing with different optics. I expect that I'll like the NEX-7 even better when it arrives, because of the better EVF and higher resolution. But I'm reserving judgement until I actually have a camera to check out.
As far as mirrorless cameras in general go, none of them have proved to be the pocketable, go-anywhere cameras that my Nikon P7100 is, but they do make a compact portable kit that can be toted around painlessly in a small bag.